How can I get the Wanut theme to remain after closing Firefox? (It keeps reverting to a default.)

I downloaded the Walnut theme from the Personas website. I enabled it after downloading. It only remains as long as I'm in the browser for that session. The next time I open Firefox, it reverts to a default theme.

I downloaded the Walnut theme from the Personas website. I enabled it after downloading. It only remains as long as I'm in the browser for that session. The next time I open Firefox, it reverts to a default theme.